1. Field of the Invention
This invention relates to an electric pressing iron having a pressing iron body portion made of silicon containing cast aluminum and equipped with an electric heating unit and with a plate-shaped soleplate made of low-silicon aluminum and secured to the pressing iron body portion in a heat-conducting relationship thereto, and to a method of manufacturing an electric pressing iron.
2. Background Information
From U.S. Pat. No. 2,846,793 there is known an electric pressing iron having an aluminum body portion to which is secured a pressing iron shoe made of carbon steel. The pressing iron shoe is coated with a nickel layer and a chromium layer. It is a disadvantage that nickel-plated and chromium-plated carbon steel fails to meet the requirements imposed on the corrosion resistance of a steam pressing iron, particularly in its steam discharge ports.
Using a soleplate made of steel is an obvious solution because of its relatively high basic hardness and low coefficient of thermal expansion determining the soleplate""s tendency to deform under the action of heat from the pressing iron. There is less likelihood, therefore, of cracks forming in a steel soleplate""s coating. On the other hand, a pressing iron with a steel soleplate has a higher power loss because, compared to aluminum, it is a poorer conductor of heat. Formability and blankability are also less good. This disadvantage is all the more aggravated by the increasing demands placed on precisely formed recesses with predefined rounded radii and the formation of holes in the soleplate.
An approach is also known which includes the step of coating the soleplate of an electric pressing iron with nickel using a plasma or flame spraying method, thereby improving the soleplate""s scratch resistance. A disadvantage of this type of coating is that it can be produced only at great outlay and generally requires mechanical pretreatment and aftertreatment by blast grinding and drag grinding in order to achieve adequate adhesion of the coating on the one hand and the required final smoothness on the other hand.
From EP 0 754 256 there is already known an electric pressing iron of the type initially referred to. This pressing iron has a body portion made of silicon containing cast aluminum, being heat-conductively bonded to a plate-shaped soleplate made of low-silicon aluminum. In this case the soleplate is anodized, as the result of which the surface of the soleplate is transformed to an aluminum oxide layer. This type of surface treatment has turned out, however; to come up against its limits as regards the level of scratch resistance and hardness that can be achieved by justifiable means.
It is an object of the present invention, therefore, to provide an electric pressing iron and a method of manufacturing an electric pressing iron of the type initially referred to, which affords the advantages of an aluminum soleplate""s good formability and thermal conductivity while at the same time satisfying requirements such as corrosion resistance, wear resistance and excellent hardness with reasonable economy of production.
As regards to the electric pressing iron, this object is accomplished in various embodiments of the invention.
According to the present invention, a soleplate made of low-silicon aluminum is used. It has proven possible to produce a coating on low-silicon aluminum by an electroplating process with reduced pretreatment requirements while at the same time achieving an optimal quality of coating. Unlike the autocatalytic chemical electroplating process which operates without external current, the present invention utilizes external current (applied to the electrodes in the electrolyte bath) to deposit in an electrolytic process metals or their alloys on the aluminum soleplate. Nickel and/or chromium, for example, provide adequate corrosion resistance as well as high hardness. A coating thickness of more than 40 xcexcm is required to prevent indentation of a naturally hard coating on the relatively soft aluminum. Advantageously, provision is made for the hardness to increase in steps or continuously from the aluminum soleplate to the outer side of the coating. It is only as the result of this outwardly increasing hardness gradient on the soleplate that each successive outer lying layer of the coating is able to display sufficient load carrying capability and partial surface compressability, ultimately enabling an excellent level of hardness and scratch resistance without the formation of any cracks in the coating.
In some embodiments, the coating is advantageously formed from one or several single layers containing pure nickel, nickel alloys with sulfur, phosphorus, cobalt, iron, sulfur and iron and/or tungsten, and/or chromium (in particular as the final coat). On account of the relatively small economic outlay involved in the deposition (using external current) of nickel or alloys it is an advantage to form a major part of the coating with these materials. Unlike pure nickel, nickel compounds or nickel alloys with sulfur, with phosphorus, with iron, together with iron, with sulfur and iron or with tungsten permit the production of layers with varying higher degrees of hardness at likewise varying corrosion resistance so that a coating structure with increasing degrees of hardness can be economically manufactured on the basis ofjust one nickel compound. It will be understood that the nickel compounds and nickel alloys referred to are presented only in terms of their main constituents and not in terms of their chemical compound. Thus, for example, the nickel-sulfur alloy used here is a nickel alloy with nickel sulfide.
In some embodiments, provision is made advantageously for a first layer of pure nickel and a second layer of a nickel alloy. Pure nickel, that is, nickel without any admixture of, for example, sulfur or phosphorus, displays high ductility as well as slightly higher hardness compared to an aluminum surface so that the tendency to form cracks under load is prevented. The initial hardness of the aluminum surface, which is typically less than or equal to 50 dphn, is increased by the pure nickel layer to more than 150 dphn. The difference in hardness between the two layers is less than or in the range of 200 dphn so that the pure nickel layer forms the first layer with load-bearing capability. The preferred choice for the second layer is a nickel-sulfur alloy, whereby a higher resistance to corrosion due to the formation of potential is achieved because, compared to pure nickel, this metal is less noble in terms of its corrosion potential. Furthermore, this nickel-sulfur alloy enables a final hardness of more than or equal to 400 dphn to be obtained so that the difference in hardness between pure nickel and nickel/sulfur is also adequate. A third layer of chromium further increases the overall hardness characteristic of the coating to approximately more than or equal to 800 dphn, resulting in excellent resistance to scratching. Having the chromium layer as the outermost layer is also an advantage in that it suffers no discoloration or tarnishing under the action of heat, which on pressing irons can be as high as 300xc2x0 C. Furthermore, the chromium layer also increases protection.
It is advantageous for the coating to be structured in its degree of hardness so that the first layer has a hardness of at least more than or equal to 150 dphn, the second layer a hardness of more than or equal to 350 dphn, and a third or outermost layer on the soleplate a hardness of more than or equal to 550 dphn, particularly more than 700 dphn. This progressively increasing hardness in the coating structure is necessary because the electroplating is performed on low hardness aluminum resulting in a layer structure which, on the whole, has adequate load carrying capability. Experience has shown that, in order to achieve excellent resistance to scratching, the differences in hardness between adjoining layers are not allowed to exceed certain limits so as to prevent the formation of cracks under thermal load. Ideally, the difference in hardness between the aluminum base material of the soleplate and the first layer should not exceed 250 dphn, the difference in hardness between the first and the second sub-layer should not exceed 350 dphn, and the difference in hardness between the second and third layer should not exceed 500 dphn in order to obtain a structure with good load carrying capability and zero tendency to form cracks. The first layer is constructed to provide only a moderate increase in hardness and is mainly optimized with a view to ductility so that any cracks which form nevertheless are certain not to extend through to the aluminum and possibly cause corrosion. The second layer is important for increasing the resistance to corrosion and for leveling the surface. The need for mechanical pretreatment and aftertreatment, such as is necessary with plasma spraying or anodizing, is thereby obviated. The outermost layer has to retain its high-quality appearance and be as hard as possible. This explains why the first layer is less hard but the outermost layer very hard. Some coatings make do with fewer different layers, uniting the above characteristics to a certain degree.
Advantageously, the outermost or third layer of the coating is a chromium layer with a hardness of between 700 dphn and 1,100 dphn. The soleplate is thus able to cope with the greatest scratch loads during ironing, including under the action of heat.
Advantageously, the coating comprises a first layer with a thickness of 10 to 70 xcexcm, particularly 50 xcexcm, a second layer likewise with a thickness of 10 to 70 xcexcm, particularly 50 xcexcm, and a third layer with a thickness of 10 to 50 xcexcm. Ideally, the first and second layer each are 50 xcexcm thick, and the third layer is 20 xcexcm thick. With aluminum having a coefficient of thermal expansion of around 24xc3x9710xe2x88x926/K, nickel a coefficient of thermal expansion of around 13xc3x9710xe2x88x926/K, and chromium a coefficient of thermal expansion of around 7xc3x9710xe2x88x926/K, the layer structure of the coating is designed with an outwardly decreasing coefficient of thermal expansion. The elongation at rupture values of the coating metals increase in the direction of the base material (aluminum) so that thermal stresses due to a bimetal effect do not produce any cracks, particularly in the first nickel layer. The layer thicknesses are optimized accordingly to ensure maximal durability of the electrodeposits. In view of the fact that coatings applied by electroplating using external current are generally thicker at the edges, for example, than in the central zones of uninterrupted surfaces, assuming that no measures are taken to optimize the primary current distribution in the interest of a uniform layer thickness, the figures quoted for the layer thicknesses apply to a central planar section of the soleplate that is not directly contiguous to any holes, edges and recesses, where present.
In an advantageous further aspect of the present invention, the coating has an overall thickness of more than 60 xcexcm. In contrast to heretofore coated soleplates made of steel, for which a coating thickness of less than 40 xcexcm has been used, a soleplate coating which is at least 40 xcexcm thick or, better still, at least 60 or 80 xcexcm thick, ensures that the high requirements imposed on an electrodeposit on aluminum are met.
The object of the present invention with regard to the method is accomplished in various embodiments of the invention.
In an advantageous further aspect of some embodiments of the invention, the soleplate to be coated with nickel by electrodeposition is immersed in an electrolyte bath with external current applied to its electrodes, wherein a screen made of a non-conductive material as, for example, plastic, is arranged in such a way that the deposited layer is uniformly distributed in its thickness over the surface of the soleplate.
For similar reasons, for electrodeposition of a chromium layer the soleplate is immersed in an electrolyte bath in such a way that a shaped anode (conforming to the shape of the soleplate) is positioned in front of the soleplate, thus resulting in the deposition of a layer having an essentially homogeneous thickness.
Further objects, advantages, features and application possibilities of the present invention will become apparent from the subsequent description of several embodiments and the accompanying drawing. It will be understood that any single feature and any meaningful combination of single features described and/or represented by illustration form the subject-matter of the present invention, irrespective of their summary in the patent claims or their back reference.